Such scales are known to comprise an elongate metallic scale member having alternate marks and spaces provided along its length. Our earlier International Patent Specification No. W088/00331,now U.S. Pat. No. 4,926,566, describes such a scale, and a method of producing it by rolling. The scale described in that specification is in the form of a flexible tape having a substrate of spring steel which is relatively inextensible. A softer copper layer is provided on the upper and lower surfaces of the spring steel substrate. The marks and spaces of the scale are embossed in one of the copper layers by an embossing roller having a corresponding profile. The tape is embossed by passing it through a nip between this embossing roller and a support roller, which has a plain cylindrical surface.
Although the structure of the tape with a relatively inextensible substrate and a soft embossing layer allows the embossing to take place while at the same time providing the scale with some stability against longitudinal extension under the roller pressure, nevertheless there are inevitably strains introduced into the tape during the rolling process, however slight, as a result of the stresses caused by the rolling. For example, the embossed surface layer will be strained more than the other surface layer, resulting in bowing of the tape. As the tape emerges from the rollers, the stresses in the tape are relieved and there can be a very slight change in the local pitch of the scale as a result. The resulting errors can be cumulative.